update documentation

[ryanraaschCDC]

it's been a while since we have added to the documentation or even reevaluated its current state.

[dylanhmorris]

Suggestion: consider using this as an opportunity to migrate to Zensical from mkdocs / material for mkdocs. See https://github.com/cdcent/cfa-predict-handbook/issues/590 (we landed on Zensical)